Tetrahydroazepine (THA) is an impurity that is formed when adiponitrile (ADN) is hydrogenated to give aminocapronitrile (ACN) and hexamethylenediamine (HMD). THA is an undesirable impurity, because it causes branching and color in Nylon 6 and Nylon 66 polymer. THA can be removed from ACN and HMD by distillation in the presence of a strong base, such as sodium hydroxide. The strong base is believed to catalyze the reaction of the THA with either ACN or HMD to form high boiling THA-containing oligomers that are easily removed from the ACN and/or HMD by distilling the ACN or HMD overhead and leaving the THA-containing high boiling oligomers as distillation bottoms. See U.S. Pat. No. 5,192,399.
According to U.S. Pat. No. 5,192,399, ACN or HMD contaminated with THA may be fed to a distillation apparatus (a single-stage flasher), along with an aqueous caustic solution. Product ACN or HMD, considerably reduced in THA content, is taken overhead, and the THA-containing oligomers are removed with the bottoms from the distillation. A single, multi-stage distillation column may be employed, but it is not necessary.
The type of distillation process described above is capable of achieving maximum ACN yields of only about 85%. While the exact chemical theory explaining this yield limitation is not well understood, it is believed that as ACN is distilled from the THA/ACN mixture, the THA/ACN oligomer is broken down, causing more THA to be taken overhead, thus contaminating the ACN or HMD product. Thus, attempts to recover more ACN and/or HMD inevitably result in an increase in THA content in the ACN or HMD product.
It would be desirable to have a process for removing THA from ACN and/or HMD to produce ACN and/or HMD in yields higher than about 85% while maintaining low THA content.